The explosions took place as students were leaving the building. Most of the victims were girls, officials said.
A bomb exploded near a girls' school in a majority Shiite district of west Kabul on Saturday, killing at least 30 people, many of them young pupils between 11 and 15 years old. The Taliban condemned the attack and denied any responsibility.
The explosions took place as students were leaving the building on Saturday, with pictures on social media showing abandoned school bags in the street.
Most of the victims were girls, a ministry of education spokeswoman said.
Ambulances evacuated the wounded as relatives and residents screamed at authorities near the scene of the blast at Syed Al-Shahda school, in the Dasht-e-Barchi neighborhood, Interior Ministry spokesman Tariq Arian said. The death toll was expected to rise further.
The bombing, apparently aimed to cause maximum civilian carnage, adds to fears that violence in the war-wrecked country could escalate as the U.S. and NATO end nearly 20 years of military engagement.
Residents in the area said the explosion was deafening. One, Naser Rahimi, told The Associated Press he heard three separate explosions, although there was no official confirmation of multiple blasts. Rahimi also said he believed that the sheer power of the explosion meant the death toll would almost certainly climb.
Rahimi said the explosion went off as the girls were streaming out of the school at around 4:30 p.m. local time. Authorities were investigating the attack but have yet to confirm any details.
While no one has claimed responsibility for the bombing, the Afghan Islamic State affiliate has targeted the Shiite neighborhood before.
Images circulating on social media purportedly showed bloodied school backpacks and books strewn across the street in front of the school, and smoke rising above the neighborhood.
At one nearby hospital, Associated Press journalists saw at least 20 dead bodies lined up in hallways and rooms, with dozens of wounded people and families of victims pressing through the facility.
The attack comes days after the remaining 2,500 to 3,500 American troops officially began leaving the country. They will be out by Sept. 11 at the latest. The pullout comes amid a resurgent Taliban, who control or hold sway over half of Afghanistan.
The top U.S. military officer said Sunday that Afghan government forces face an uncertain future and possibly some “bad possible outcomes” against Taliban insurgents as the withdrawal accelerates in the coming weeks.